The invention relates generally to time division multiple access communication systems, and it relates, in particular, to monitoring the input power to a transponder of a communication satellite system.
Communication satellites are usually designed to operate with a large number of simultaneous users so that they can receive and retransmit independent and concurrent messages. In the past, the conventional method of handling concurrent messages has been frequency division multiple access (FDMA). In an FDMA system, different channels or communication paths are assigned a different frequency within the bandwidth of the satellite transponder. The satellite is equipped with a wide band receiver and transmitter, such as a travelling wave tube (TWT) which would receive any signal within the wide band and, after a frequency shift, retransmit the signal to the ground stations at a considerably higher power level. The transfer characteristics of a TWT are illustrated in the graph of FIG. 1 in which the input RF power is plotted on the horizontal axis and the output power is plotted on the left vertical axis. The phase change is plotted on the right vertical axis. The output power or amplitude as a function of input power is shown by curve 10 which has a linear portion at lower input power before reaching a saturation peak 12. Both the input and the output power are measured as a back-off from the values at the peak 12. The phase characteristics are shown in curve 14.
A satellite is almost always power limited so that it is desirable to operate at the saturation point 12 in order to obtain maximum output power. However, near saturation, the TWT is very non-linear. A non-linear amplifier causes multiple signals of different frequencies to mix, thereby producing cross-talk between the signals. Therefore in an FDMA system, it is necessary to back-off the input power to a linear portion of the amplitude curve 12 where the operation of the TWT is much more linear. This input back-off, of course, incurs a substantial power penalty.
Some of the recently introduced satellite communication systems use time division multiple access (TDMA). A TDMA communications network is schematically illustrated in FIG. 2 for three ground stations 16, 18 and 20, communicating with each other through a communications satellite 22. The ground stations 16-20 are intended to be remotely located and to be operating with a minimum of local control. A network monitoring station 24 performs a variety of monitoring and supervisory tasks, some of which will be described later. The ground stations 16-20 are time synchronized with each other and each is assigned a particular repeating slot for transmission to the satellite 22. At the input of the satellite 22 the up-link transmissions are interleaved into a serial stream. The satellite changes the carrier frequency of the signal, amplifies the signal and retransmits the serial stream to all of the ground stations 16-20, including the network monitoring station 24, with a minimal change in the content of the signals.
The TDMA data format is illustrated in FIG. 3. Frames repeat on a fixed period, e.g., 2 ms. Each frame is divided into N slots or bursts. The first slot in a frame may be assigned to some housekeeping duties. The remaining slots in each frame are assigned to particular ground stations. Each of the slots is divided into several sequential portions. The first portion is a carrier recovery segment which allows the recovery by a receiving ground station of the precise carrier frequency used by the ground station originating the transmissions for that particular slot. A second portion is the clock recovery segment which allows recovery of the data clock of the transmitting ground station. The carrier and clock recovery segments are collectively called the preamble of the burst. The clock recovery is followed by a unique word which is then followed by the data being transmitted. The data portion has a fixed length for a particular slot but a fraction of the data portion may be vacant because there is no further data to transmit. However, there may be a requirement that data occupy a minimum fraction of the data portion. Finally there is a guard space between slots to allow for equipment variations in the ground stations and also to account for changing path lengths to the satellite. The ground stations 16-20 and the monitoring station 24 need to be synchronized to the extent that transmissions do not overlap, that a ground station 16-20 knows when its own burst occurs, and that the boundaries of the TDMA format are generally known. Because the transmissions are interleaved at the satellite 22, the ground stations need to adjust their own synchronization to that at the satellite taking into account the finite propagation times to and from the satellite.
In TDMA systems, only a single carrier frequency is present at the input of the TWT at any instant so that non-linearity and intermodulation are not problems. Accordingly, the TWT can be operated close to the saturation point 12 of the TWT shown in FIG. 1. Operating at saturation can provide a ten-fold increase in down-link power over FDMA systems, thus increasing the DC to RF conversion efficiency of the satellite.
The hardware of a satellite system is illustrated in more detail in FIG. 4. The ground stations 16, 18 and 20 interleave their transmissions to the satellite 22, which receives all these transmissions on a receive antenna 25. A receiver 26 preamplifies and frequency shifts the received signals. It is common for TDMA systems to operate with several frequency-separated channels. Each channel follows the TDMA format of FIG. 3 although the parameters of the format may differ between channels. the output of the receiver 26 is divided according to frequency to power amplifiers or TWTs 28, 30 and 32 for each of the channels. After power amplification, the signals are recombined and retransmitted through a transmit antenna 34 back down to the ground stations.
Each of the TWTs 28-32 should be operated near its saturation point 12 as shown in FIG. 1. The operation point is determined by the carrier power transmitted by the ground station 16, 18 or 20 for the respective slot. If the carrier power is significantly lower, there is input back-off to the left in FIG. 1 (also called underdrive) with the result that the output power is also reduced. An underdrive condition decreases the uplink carrier-to-noise ratio and the overall bit-error performance of that channel may be degraded by spill-over from unattenuated adjacent channels. On the other hand, overdrive to the right of the saturation point puts the TWT into a highly non-linear region of operation. This non-linearity tends to further spread the spectrum of the band-limited digital signals into the bands occupied by adjacent channels and thereby causes cross-channel interference. It furthermore may degrade the bit-error performance because of bit stream crosstalk and intersymbol interference caused by bandwidth filtering of the non-linear distorted signals. In some situations, it is desirable to maintain operation in a slight overdrive condition so that additional input back-off introduced by propagation path uncertainties such as rain and humidity will bring the operating point to a slight underdrive condition with a negligible change in the output power. In this case, however, it is necessary to tightly control the operating point so that the overdrive condition does not become extreme, thereby introducing unacceptable non-linearity. Therefore, monitoring the satellite transmitter input drive level and the carrier-to-noise ratio of each burst or slot would be useful in adjusting the burst power transmitted to the satellite, in identifying problems associated with each earth station, and in monitoring any performance degradation of the satellite subsystems.
One method of measuring the input back-off relies upon the non-linearity the TWT in the vicinity of saturation. A low-level CW-carrier or pilot signal is transmitted from one of the ground stations or the monitoring station. The pilot has a frequency within the band of the transponder but offset sufficiently from the center of the band and of sufficiently low power so as not to interfere with the data signal. A measurement is then made at the monitor station of the power level of the pilot signal retransmitted by the TWT in the absence of a data carrier during the guard space. Then data transmission begins from the ground station assigned to the slot following the guard space but the transmission of the pilot signal retransmitted by the TWT continues at the same power level. The monitor station then reexamines the power level of the retransmitted pilot signal. Because of the non-linearity of the TWT, there is intermodulation between the pilot and the data modulated carrier which reduces the signal intensity of the pilot signal. This reduction or suppression of the pilot is a constant function of the input back-off. The TWT can be characterized beforehand by measuring the retransmitted pilot signal levels in the absence of a data carrier and in the presence of a measured data carrier input power. If the saturation peak 12 of the input carrier power is determined by another set of measurements on the carrier input and output powers, then the relation between pilot suppression and input back-off can be characterized by a single set of tests, for instance, on the ground before the launch of the satellite. Then, with the satellite in orbit, a measurement of the pilot suppression can be used to measure the input backoff of the carrier to the TWT.
This method has been used in a Satellite Business System (SBS) satellite using demand assigned TDMA. The pilot has been put within the satellite transponder bandwidth of 43 MHz at a frequency 20 MHz below the center frequency of the channel. Because the transmission rate of the TDMA channel is 24 MSps (megasamples per second), the bandwidth-to-transmission rate is sufficiently high to allow the transmission of a measurable pilot without interfering with the data transmission. Since the data signal at this offset frequency is approximately 15 dB below the peak, a narrow pilot filter at the monitor station can be used to measure the pilot power in the presence of the data modulated carrier. This system requires that the data portion of the slot be sufficiently long to allow detection of the suppressed pilot. In the SBS system, the transmitted data is required to be a minimum of 50 .mu.s and this condition is satisfied for quadrature phase-shift key (QPSK) transmission.
A more recent Intelsat system utilizing TDMA has operational characteristics that are different from the SBS approach. In the Intelsat system, the trandponder bandwidth is 72 MHz and the transmission rate is 60 MSps so that the spectral power density of the data signal anywhere within the transponder bandwidth is no lower than 6 dB below the peak, that is, at least 9 dB higher than the roll-off in the SBS system. If the pilot filter bandwidth is further reduced to compensate for this 9 dB rise of the data carrier in the vicinity of the pilot signal, the time period required to measure the pilot power becomes excessively long. This problem is worsened because in the Intelsat system the minimum length of data in the data segment is reduced from that in the SBS system to a minimum length of 4 .mu.s.
Another useful parameter, besides input power back-off, to be measured in TDMA systems is the carrier-to-noise ratio which will give an indication of the expected bit-error rate on a particular channel and slot. The noise measurement must account for the fact that noise during data transmission is suppressed by a TWT operating near saturation.